Scientists have developed a revolutionary new treatment for neurological diseases that uses an injection to tweak the way genes work in the brain.

The research raises hopes for a new era of effective treatments for some of the most debilitating - and so far incurable - brain conditions, including cancer and Alzheimer's disease.

Tests of the therapy at Harvard Medical School in Boston found that a simple injection was able to cure mice of a potentially fatal brain disease. The researchers behind the breakthrough are planning further tests and expect to conduct human trials within five years.

The team used a powerful new technique called RNA interference to silence faulty genes or viruses that cause brain diseases. The principle of gene silencing is simple: scientists build tiny strands of the genetic material called RNA which, when injected into cells, latch on to problematic genes and smother them, effectively shutting them down.

Until now, attempts to use gene silencing to treat brain diseases have been severely hampered for two reasons. First, many drugs injected into the body are barred from getting into the brain by a membrane that protects it from dangerous viruses and microbes in the bloodstream. Because of this, most gene-silencing treatments require injections directly into the brain - a second serious drawback because it involves a dangerous surgical procedure and only delivers the treatment to the cells at the end of the needle.

The Harvard team, led by Manjunath Swamy, found a simple way around these problems. They made strands of therapeutic RNA in the lab and mixed them with a tiny part of the rabies virus which helps it get into the brain and infect cells. The key-like fragment is harmless on its own.

The researchers squirted some of the mixture on to a dish of nerve cells and found that the strands of RNA they had created were smuggled inside the cells, where they shut down certain genes.

Next, the team tested the therapy on mice that had been infected with a fatal brain disease, viral encephalitis. The scientists found that regular injections of RNA mixed with the rabies virus fragment shut down genes in around half of the cells in the animals' brains and stopped the disease from spreading. Of the mice that received injections, 80% were cured, while all of the mice that were untreated died, the team report in Nature.

"We expect this work to move quite fast now, because there is so much interest in this kind of treatment. Potentially, it could be used for a wide variety of neurodegenerative diseases, but also for controlling cancers that spread to the brain," said Professor Swamy.

Therapies based on RNA interference have become the next great hope for medicine, and a large number are either in or about to start early clinical trials in humans. The technique earned its discoverers, the US researchers Andrew Fire and Craig Mello, the Nobel prize in medicine or physiology last year.